A little weekend entomology

I’ve just come back from a weekend at my parents’ house, celebrating my Dad’s birthday and enjoying the beautiful early summer weather. My parents live on the Oak Ridges Moraine in Ontario – they have a gorgeous piece of property they’ve named Hawksview because of the panoramic vistas you get from on top of a large pile of old glacier scrapings.

View from my parents’ house – on clear days you can see all the way to Lake Ontario. Photo: Kathleen Timms

Coming from my shamefully barren urban yard, I am always amazed at the diversity of insect life on my parents’ property. Saturday morning I went outside with a cup of coffee to sit and enjoy the gardens, and within minutes was amazed at the amount of flower visiting taking place in front of me. I did a quick and unscientific count and came up with at least four species of big bees, six species of butterfly and who knows how many smaller flies and sundries buzzing in and out of the Weigela, Salvia, and Allium flowers.

My husband has done research on Bayesian learning in bumblebee foraging, and so the two of us often get caught up in watching bees drink nectar and thinking about their decision-making. As we were doing so this time, he noticed that one of the bee species was acting as a nectar robber – in other words, it was cutting a hole in the bottom of the flower and drinking the nectar through this hole instead of entering the flower in the usual way.

A carpenter bee, Xylocopa virginica, sitting on a Weigela flower and taking nectar through a hole it has cut in the base of the corolla. Photo: Laura Timms

This kind of interaction is referred to as nectar robbing because the bee is getting what it is after – the nectar – without paying the price of taking pollen along with it to the next bloom. Nectar robbing is often used as an example of how a mutualism like pollination, where both parties are supposed to benefit, can be subject to cheating. Charles Darwin speculated on the harm that nectar robbing must cause to plant fitness in his book The effects of cross and self-fertilisation in the vegetable kingdom (1877). I have to admit I didn’t know much about nectar robbing beyond the basics, and I also didn’t know what the species of bee was doing the robbing. A quick online search gave me answers on both counts.

The bee had a big, shiny black abdomen and a black spot on a fuzzy yellow thorax – had to be the eastern carpenter bee, Xylocopa virginica. It turns out carpenter bees are among the most common nectar robbers out there – they have short tongues, and can’t reach the nectar in flowers with long corollas, like my mother’s Weigela. But, it also turns out that nectar robbing isn’t necessarily always cheating, and may not be bad for the plant. Maloof and Inouye (2000) reviewed the literature on nectar robbing and found that there was more evidence for positive or neutral effects of nectar robbing on plant fitness than for negative effects. This is because other pollinators may still visit robbed flowers, some nectar robbers do actually pollinate, and nectar robbing can actually result in greater amounts of pollen flow between different plants and thus increase outcrossing. Fascinating!

I passed on this information to my parents, and resumed my garden sitting and coffee drinking. My attention was soon diverted again, this time by a bright red beetle on my Mom’s lilies. I didn’t need the internet to identify this beetle – it is an old friend of mine from when I worked in Switzerland at the Commonwealth Agricultural Bureau International. The lily leaf beetle, Lilioceris lilii, is an invasive species in North America and a voracious consumer of lilies. While the adult beetles are quite attractive the larvae have the gross habit of carrying around their frass on their backs, using it as a shield to deter predators and parasitoids (which is not always effective – see Schaffner and Müller 2001 for example).

I started scanning the lilies for beetles and larvae and removing them by hand – by far the best control method for a home gardener. I started squishing beetles and tossing them aside, when I remembered a recent email from a graduate student at the Université de Montréal. Alessandro Dieni is a student in Jacques Brodeur’s lab, and his research involves reconstructing the path of invasion of the lily leaf beetle using population genetics. Alessandro is looking for samples of the beetle from all over North America for his analysis, and so I stopped throwing the beetles away and started putting them in a jar of rubbing alcohol – the best collecting supplies I had on hand. I included the larvae too, after removing their fecal shields (for which my Mom made me wash my hands outside before coming in the house). It turns out that Alessandro can only use adults for his analysis, so the larvae aren’t much help. If you have lilies and have noticed these beetles in your garden, Alessandro would appreciate samples of adult beetles. You can contact him at alessandro.dieni-lafrance@umontreal.ca, and he will send you all the information you need, including a kit for collecting and preserving them.

One of the side effects of being an entomologist is being frequently asked the question: “What is this on my plant?” My dad asked me a few weeks ago about some galls he had noticed an oak tree, but I told him I couldn’t help him much without seeing them. So, one of my final tasks of the weekend was to check out the tree. This is what I saw:

Galls on a red oak, Quercus rubra, tree. Most are at the base of a branch. Some of the galls have had lots of adults emerge (note the emergence holes), and some have not. Photo: Laura Timms

My basic knowledge of oak galls told me that these galls were probably caused by cynipid wasps, but I wasn’t sure. We cut one open, and sure enough there was an almost fully developed wasp inside a chamber. Gall wasps lay their eggs in plant tissue, and the presence of the eggs induces the plant to produce the special types of highly nutritious cells that make up the gall. Larvae feed in chambers inside the gall, pupate, and then emerge out of small holes like the ones in the picture. I haven’t gotten very far with the identification of exactly which species of wasp is affecting my parents’ tree, although I’ve promised to look into it further and let them know if their tree is in serious trouble. I’m also curious to know if there are any other species inside the gall – oak galls are a fascinating system for work in community ecology, with a cast of cynipid wasps, parasitoids, predators and inquilines (e.g. Stone et al. 2002).

I’ve always said that my parents’ place would be a great field station. I’ve only mentioned three of the ecological tidbits that caught my eye this weekend, but I could go on about the way that dog-strangling vine is taking over the meadow and forest floor, our observations of caterpillars brought to the nest by purple martins, or the cool moths that show up at night by the outside lights. For the sake of brevity, I think those will all have to wait. In the mean time, now that my weekend entomology is over, I’m going to return to my regularly scheduled entomology and hit the microscope!

Literature cited

Darwin, C. 1877. The effects of cross and self fertilisation in the vegetable kingdom. D. Appleton and Co., New York.

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